I've been using the season table built into the apex controller. It varies throughout the year a few degrees. I think today's forecast is 75.9 on the reef.
I've found lower temps have helped me keep corals I had trouble with in the past (eg yumas, rics).
My tank ranges from 75F to 79F without running the heater (even now, in the winter). I like to see it in the 76F area myself. Water chemistry seems to guide my coral growth. Don't think temp would matter much - unless it dropped below 70F or rose above 80F.